Barclaycard's Web site has buckled under the weight of integrating newly acquired Goldfish cardholders, leaving customers unable to log in to access their accounts online.
Barclaycard is currently migrating 1.7 million cardholders from the Goldfish credit card business it acquired from Discover Financial Services earlier this year.
This increased traffic has contributed to customers experiencing problems accessing their accounts online.
In a statement, a Barclaycard spokesperson says: "We are currently experiencing high demand for the service due to the migration of Morgan Stanley and Goldfish customers to Barclaycard. Whilst we, of course, planned for this increased demand the volume has been much higher than anticipated."
Furious cardholders have taken to forums, complaining they have been unable to log in for up to two weeks and criticising the company for failing to explain the problem.
Barclaycard increased the site's capacity on Friday, but a message to customers attempting to log in still warns: "We are currently experiencing high demand, which may affect the speed of the service."
Barclays customers also had trouble accessing their accounts online last week after problems with the bank's Web banking platform, which is separate from the Barclaycard system.